The invention relates to a method for the production of medical fittings, in particular dental fittings, in which a workpiece, from which the fitting is produced, has material removed from it by machining with a tool.
A method of this type is known from EP-A-0,182,098, for example. A workpiece made of dental ceramic material is arranged on a holder which advances with rotation as the machining is carried out. In doing so, the workpiece is alternately machined by a cutting wheel or a finger-shaped grinder rod which are arranged on a tool carrier. The height position of the tool carrier is controlled by a program in such a way that the shaping by material removal is effected along the intended contour of the fitting. The contour to be formed has been obtained in advance from a visual scan of the pretreated tooth into which the dental fitting is to be implanted.
A method for the production of dental prosthetic fittings is also known from EP-A-0,455,853. Here, the machining is effected simultaneously by two tools mounted on different tool heads.
A problem during machining of the workpiece is that the removal capacity of the tools is in most cases not constant along the intended contour of the fitting. A great deal of material is removed at some points of the contour, in particular if a fitting with steep flanks or sizeable depressions is to be produced. At other 5 points, by contrast, the amount of material removed can be very slight, e.g. if a flat, smooth surface is to be obtained.
Too high a removal capacity can lead to overheating the tool or even to its destruction. It is therefore highly important to ensure that a maximum loading of the tools is not exceeded throughout the machining of the workpiece.
For this purpose, it has been proposed, with a constant frequency of rotation of the rotating tool, to constantly monitor the power consumption of the rotary drive mechanism. This power consumption then represent a measure for the instantaneous loading of the tool. If the power consumption exceeds a defined maximum value, the speed of advance between workpiece and tool is reduced until the power consumption once again falls below the maximum value.
It is true that satisfactory control can in many cases be achieved using such a method. However, this control is often not sufficient to reliably avoid power peaks, which can lead to greatly increased wear of the tool. Such power peaks can occur particularly upon rapid change over between a low removal capacity and a high removal capacity. This is often the case, for example, when producing sharp-edged structures with steep flanks, as are often found in tooth crowns.
The object of the invention is therefore to provide a method for the production of medical fittings, in which method load peaks of the tools can be effectively avoided and the fitting can be finished in the shortest possible time.
This object is achieved by means of a method for the production of medical fittings, in particular dental fittings, in which a workpiece, from which the fitting is produced, has material removed from it by machining with a tool, the tool and the workpiece executing a relative movement along a trajectory, and the actual loading of the tool being used to control the speed of the relative movement. According to the invention, the expected loading of the tool along the trajectory is calculated and is used to control the speed.
As the expected removal capacity of the tool is already calculated in advance, even very rapid load changes can be detected in good time and taken into account in controlling the movement between workpiece and tool. In this way, power peaks are effectively ruled out from the outset. Whereas, for example, a low speed between tool and workpiece is chosen in advance for those parts of the trajectory in which a high removal capacity is to be expected, the movement in regions with only slight removal can be very fast. This leads to a considerable saving in time during production of the fittings. Moreover, the forces between tool and workpiece are reduced.
This method can be used to particular advantage when the tool is essentially cylindrically symmetrical and rotates about its axis of symmetry by means of a rotary drive mechanism. The tool is advantageously an essentially cylindrical grinder rod with a circumferential surface and an end surface.
There are two possibilities in particular for determining the instantaneous loading of the tool. According to the first possibility, the measure for the actual loading of the tool is the frequency of rotation of the tool determined at an essentially constant power consumption of the rotary drive mechanism. According to the second possibility, the measure for the actual loading of the tool is the power consumption of the rotary drive mechanism determined at a predetermined frequency of rotation.
The types of relative movement between the tool and the workpiece are often defined by the device with which the fitting is produced. The method according to the invention can be used to particular advantage if the relative movement between the tool and the workpiece includes the following types of movement:
a) linear feed between the tool and the workpiece along a feed direction which is essentially perpendicular to the axis of symmetry of the tool;
b) change of depth of the tool in the workpiece essentially along the axis of symmetry of the tool; and
c) one-dimensional sideways movement between the tool and the workpiece along a predetermined path in a plane which is essentially perpendicular to the axis of symmetry of the tool and is different than the direction of the linear feed.
The one-dimensional sideways movement preferably consists of a swiveling of the tool about an axis which is essentially parallel to the axis of symmetry of the tool. However, another possibility is a transverse movement of the tool along a direction which is essentially perpendicular both to the axis of symmetry of the tool and also to the direction of the linear feed.
In order to be able to form complicated structures from the workpiece, it can be expedient if the relative movement between the tool and the workpiece moreover includes a pivoting of the workpiece about an axis which is essentially parallel to the direction of the linear feed.
The desired contour is advantageously obtained from the workpiece by the linear feed taking place in steps in the same direction throughout machining. The feed in this case preferably takes place in steps which are small compared to the mean diameter of the tool. The trajectory is divided into sections between which no linear feed takes place. The expected loading for each section is then calculated by comparison of the trajectory along this section and the trajectory of the respective previous section. Each movement section can be made up of said movement types b) and c), that is to say a change of depth of the tool and a one-dimensional sideways movement. The expected removal capacity can be calculated at least one previously chosen point of the movement section.
If the tool is a cylindrical grinder rod, it is advantageous to calculate the expected loading in the following manner: In order to determine a measure for the expected loading upon a change of depth of the grinder rod in the workpiece, that part of the end face is approximately determined from which material is removed. The speed of the change of depth of the tool in the workpiece is then limited to a maximum speed which is determined by reference to a table in which, for several values of that portion of the end face from which material is removed, the associated maximum speed is stored.
The one-dimensional sideways movement is controlled in a similar way. In order to determine a measure for the expected loading upon the one-dimensional sideways movement, the height of that part of the circumferential surface of the grinder rod from which material is removed is calculated. The speed of the one-dimensional sideways movement is then limited to a maximum speed which is determined by reference to a table in which, for a number of values of the height of that portion of the circumferential surface from which material is removed, the associated maximum speed is stored.